The Essential Steps of Plaster
For all but the most daring of plasterers, we recommend protection of all surfaces that do not wish to be coated in lime. This includes all exposed doors and windows, trim, floors, and anything else in the vicinity.
Mixing plaster involves interacting with both dry and wet lime. Proper Personal Protective Equipment (PPE) is important, and includes goggles, respirator, and gloves. It’s best to keep an eyewash station, vinegar, and lotion on-hand, just in case.
When making plaster, it’s important to make sure that the aggregate and the lime are mixed thoroughly.
Hydraulic limes should be mixed as dry powder with the aggregate, then add water. A little water can go a long way, so add little bits at a time.
Hydrated limes benefit from pre-wetting. You’ll mix lime powder with water to a pudding consistency called lime putty. Type S limes can be used the next day, whereas high calcium limes should be kept in this state for 3 months or more prior to mixing with aggregate.
Recipes generally call for 2.5 - 3 parts aggregate to 1 part lime powder or lime putty for the base coats, and tend to be more lime-rich for the finish coat.
3 : 1 sand : lime or 2.5 : 1 sand : lime
With plaster, you’re looking for a mortar consistency - too dry and it won’t spread, too wet and it won’t have body.
Solid Substrate
A solid substrate is an essential foundation for lime plaster. In order to pass as SOLID, it must meet 3 criteria:
A- Absorbent - must be able to absorb (and release) moisture.
R- Rigid - must resist movement or displacement.
T- Textured - must have keys for plaster to cling to.
Prior to plastering, a 4th criteria must be met.
S- Saturated. Wet to dripping for 1st round, and wet to a sheen just before plastering.
Using a mist nozzle on a hose or a hand-pump sprayer, saturate the walls several hours before plastering, and again just before adding material to the wall.
ARTS Courtesy Sigi Koko, Lime Essentials
Getting Plastered
Start at the top, and work your way down the wall. This way, you won’t mess up the work that you’ve done later by dropping plaster onto it.
At this point, you are simply getting plaster onto the wall, smearing so that its connecting with the substrate. You’re aiming for an even thickness when applying, slightly thicker than your final thickness. This added plaster will fill in any shallow areas in the smoothing process.
Here you can see trowel lines where Catalyst has transferred material from the trowel to the wall. The wall will be smoothed after more material is on the wall to work with.
Once there is enough material on the wall to smooth, use long evenly-pressured strokes to even out the material and remove the trowel lines.
Sigi describes the process of plastering like landing an airplane, then taking off again. The runway is the wall.
Using a broad sweeping motion of your arm, starting at one end and continuing across your body, bring your trowel (airplane) in to the runway (wall) easily, until you are in contact with the plaster. Once you are fully in contact, apply pressure. You will do this for most of the sweep of your arm.
Near the end of your arm span, gently release pressure, then gently lift away from the plaster as you continue the sweeping motion.
Repeat this process from right to left, from left to right, up and down, down to up, and across like an X. This will provide the most consistent and smooth wall surface. The more you do, the more smooth and flat the wall will be.
When plastering around objects (like windows), treat the object like an edge, and work it carefully. Plaster doesn’t stick to wood framing (wood is missing the T for texture in the ARTS substrate criteria) so must be covered with something like burlap or fiberglass mesh to provide a mechanical key.
It is best practice to support the plaster at the corners to keep them from cracking. Windows and doors exert a lot of force on the framing when they are open and shut, and this movement can transfer into the plaster and show up as cracks. Use either alkali-resistant fiberglass mesh or natural fibers to resist this cracking in the corners.
Once the plaster has been applied and smoothed over the entire wall span, the finish can be applied.
The finish will depend on the plaster layer you are applying:
Scratch Coat (bottom layer in a 3 coat system) - will have horizontal scratches approximately 1/8” deep by 1/4” wide.
Float Coat (middle layer in a 3 coat system or bottom layer in a 2 coat system) - will have a textured appearance from using a sponge to finish
Smooth Coat (final coat in either a 2 or 3 coat system) - Can be perfectly smooth or have texture, depending on client requirement/plasterer skill.
This is the Float Coat for a 2 coat interior finish, which has been roughed up with an open sponge float using a circular pattern.
Once the plaster is on the walls, it should be kept wet while it cures to protect it from drying out too quickly.
Hydrated Lime Walls (Type S and High Calcium Limes) - Keep wet for 10 days.
Hydraulic Lime Walls (NHL 2.0 - 5.0) - Keep wet to 3 to 5 days.
Burlap and Canvas “Curtains” protect exterior lime plaster while it cures.
Keep direct sun, wind, and rain off the walls during the curing process. For exterior walls, we recommend burlap sheets or canvas tarps to protect the lime walls during the curing process. These can be removed once the wetting requirements for each lime type have been met. However, new lime plaster should not freeze for 28 days after application, so these curtains can protect the walls in case of a light frost.